rfc2171.txt

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Network Working Group                                       K. Murakami
Request for Comments: 2171                                  M. Maruyama
Category: Informational                                NTT Laboratories
                                                              June 1997

       MAPOS - Multiple Access Protocol over SONET/SDH  Version 1

Status of this Memo

   This memo provides information for the Internet community.  This memo
   does not specify an Internet standard of any kind.  Distribution of
   this memo is unlimited.

Authors' Note

   This memo documents a multiple access protocol for transmission of
   network-protocol datagrams, encapsulated in High-Level Data Link
   Control (HDLC) frames, over SONET/SDH.  This document is NOT the
   product of an IETF working group nor is it a standards track
   document.  It has not necessarily benefited from the widespread and
   in depth community review that standards track documents receive.

Abstract

   This document describes the protocol MAPOS, Multiple Access Protocol
   over SONET/SDH, for transmitting network-protocol datagrams over
   SONET/SDH.  It focuses on the core protocol -- other documents listed
   in the bibliography may be referenced in conjunction with this
   document to provide support and services for protocols at higher
   layers.

1. Introduction

1.1 SONET/SDH

   The Synchronous Optical Network/Synchronous Digital Hierarchy
   (SONET/SDH) [1][2][3][4] family of ITU-T standard protocols are
   designed to provide common, simple, and flexible interface for
   broadband optical fiber transmission systems.  It enables direct
   octet-synchronous multiplexing of lower rate tributaries.
   SONET/SDH-compliant transmission systems are widely deployed by
   telephone carriers world wide.

   This document defines the MAPOS protocol -- a method for transmitting
   HDLC frames over SONET/SDH. The protocol provides multiple access
   capability to SONET/SDH, an inherently point-to-point link. This
   enables construction of seamless networking environment using
   SONET/SDH as transmission media for both LAN and WAN.



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RFC 2171                         MAPOS                         June 1997


1.2 Possible Configurations

   The MAPOS protocol provides multiple access, broadcast / multicast-
   capable switched LAN environment using SONET/SDH lines as
   transmission media.  Possible configurations of MAPOS system are
   shown in the following diagrams.  In (a), two end nodes are connected
   to each other.  Figure (b) shows a star-topology "SONET-LAN" where
   multiple end nodes are connected to an HDLC frame switch. The frame
   switch forwards packets between nodes and provides multiple access
   capability. In (c), multiple frame switches are linked together,
   creating a switching cluster.


           +------+                                +------+
           | Node +--------------------------------+ Node |
           +------+                                +------+

                    (a) Point-to-Point configuration

































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RFC 2171                         MAPOS                         June 1997


           +------+                                +---------------+
           | Node +--------------------------------+               |
           +------+                                |               |
                                                   |               |
           +------+                                |               |
           | Node +--------------------------------+               |
           +------+                                |               |
                                                   | Frame Switch  |
           +------+                                |               |
           | Node +--------------------------------+               |
           +------+                                |               |
                                                   |               |
           +------+                                |               |
           | Node +--------------------------------+               |
           +------+                                +---------------+

                 (b) Point-to-Multipoint configuration


           +--------+                      +--------+
           | Frame  +----------------------+ Frame  |
           | Switch +--------+    +--------+ Switch |
           +--+-----+      +-+----+-+      +--------+
              |            | Frame  |                      +--------+
           +--+-----+      | Switch |      +--------+      | Frame  |
           | Frame  |      +-----+--+      | Frame  +------+ Switch |
           | Switch |            +---------+ Switch |      ++-------+
           +-------++                      +--------+       |
                   |________________________________________|

                  (c) Switching cluster configuration

                   Figure 1. Possible configurations

   Each port on a switch has an unique identifier within the switch. A
   node connected to a switch port must inherit the address of the port.
   That is, the node address is equal to the port identifier and is
   unique within the switch.

   In a switch cluster, a node address is subnetted. The high-order
   bits, the part where the corresponding bits in the "subnet mask" are
   1, indicate the switch address.  The remaining low-order bits
   indicate the unique node address within the switch. The two fields
   form an unique address for a given node.

   In either case, the address may be configured manually into a node
   interface, or automatically by the address assignment mechanism
   described in [5].



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RFC 2171                         MAPOS                         June 1997


   Note that any two components may be connected either directly, or via
   a long-haul SONET/SDH leased line.

1.3 Packet Transmission

   The protocol is connection-less -- when a node wish to communicate
   with some other node, it simply fills-in the destination address of
   an HDLC frame, places it in one or more SONET/SDH payloads, and sends
   it over a SONET/SDH link.

   The switch forwards the frame to its destination based on the
   destination address. In a switch cluster, the frame may be forwarded
   by multiple switches and is eventually delivered to the specified
   node.  Broadcast and multicast are also supported. Frames with an
   invalid destination address are silently discarded.

   Like ethernet, the multiple access capability is provided by a switch
   or a switch cluster. Since MAPOS is a link layer protocol, it is
   independent of the upper layer protocols. That is, it can support any
   network layer protocols such as IP. MAPOS IPv4 support is described
   in [6].

2. Physical Layer

   This protocol treats the underlying end-to-end SONET/SDH transmission
   link as if it was a plain, transparent channel.  It sends HDLC frames
   in SONET/SDH payloads, and expects them to arrive at the other end
   unaltered.

   Each node and switch should terminate SONET/SDH overhead such as
   section overhead, line overhead, and path overhead according to the
   specification of SONET/SDH. Unfortunately, SONET and SDH overhead
   interpretations are not identical. In addition, some SONET/SDH
   implementations utilize some overhead bytes in proprietary manner.

   The detail of the interpretation is beyond the scope of this
   document.  Appendix A describes some of the most significant
   differences among SONET, SDH, and their implementations that often
   causes interoperability problems.  Implementors of SONET/SDH
   interfaces are strongly encouraged to be aware of such differences,
   and provide workaround options in their products.










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RFC 2171                         MAPOS                         June 1997


3. Data Link Layer

3.1 HDLC Frame Format

   MAPOS uses the same HDLC-like framing as used in PPP-over-SONET,
   described in RFC-1662[7].  Figure 2 shows the frame format.  Logical
   Link Control (LLC), and Sublayer/Sub-Network Access Protocol (SNAP)
   are not used.  It does not include the bytes for transparency.  The
   fields are transmitted from left to right.

           +----------+----------+----------+----------+
           |          |          |          |          |
           |   Flag   | Address  | Control  | Protocol |
           | 01111110 |  8bits   | 00000011 |  16 bits |
           +----------+----------+----------+----------+
              +-------------+------------+----------+-----------
              |             |            |          | Inter-frame
              | Information |    FCS     |   Flag   | fill or next
              |             | 16/32 bits | 01111110 | address
              +-------------+------------+----------+------------

                        Figure 2.  Frame format

     Flag Sequence